In a glass producing apparatus, such as a vacuum degassing apparatus, refractory bricks are used as the constituent material for a conduit for molten glass made of a hollow pipe in some cases. As the refractory bricks, fused cast refractories have been normally used because of being excellent in heat resistance and corrosion resistance to molten glass.
When fused cast refractories are used to fabricate a conduit for molten glass, it is, however, impossible to fabricate the conduit as an undivided hollow pipe having no joints. For this reason, a plurality of fused cast refractories, which are formed in a doughnut shape having an opening in a central portion, are prepared and stacked one another to fabricate a hollow pipe, for example. With regard to such fused cast refractories formed in a doughnut shape, even in cases where fused cast refractories formed in a doughnut shape having no joints are used, it is usual that a plurality of fused cast refractories, which are formed in a substantially fan-shape or wedge shape, are prepared, and these fused cast refractories are assembled to one another along their circumferential direction to be formed in a doughnut shape.
This leads to that when using fused cast refractories to fabricate a conduit for molten glass, joints inevitably exist between adjacent fused cast refractories on the inner side of the hollow pipe, i.e. a flow path in direct contact with molten glass. It is considered that fused cast refractories are less susceptible to the oozing-out of molten glass from joints in comparison with fired bricks because of having a dense composition with a low porosity. It is, however, difficult to completely prevent molten glass from oozing out of a joint.
As one proposal, it is considered that a joint material is embedded into each of the joints between adjacent fused cast refractories forming a flow path in direct contact with molten glass. The joint material in direct contact with molten glass is, however, likely to be corroded by molten glass in comparison with the fused cast refractories because the joint material usually has a poor density in comparison with fused cast refractories. This has caused a problem in that although fused cast refractories are less susceptible to corrosion, the joints between adjacent fused cast refractories are more intensively corroded. Although it is possible to delay the oozing-out of molten glass from joints in comparison with a case where no joints are buried, molten glass oozes out of joints when the joints have been corroded.
A conduit for molten glass is equipped with a backup (supporting structure) therearound. The backup is helpful to press the conduit toward the center thereof such that the joints between adjacent fused cast refractories assembled together in a doughnut shape are brought into close contact with the adjacent fused cast refractories. Further, the backup has a thermal insulating and reinforcing function for the conduit.
As the backup, refractory bricks and solid thermal insulating materials are usually used. As the refractory bricks, fired bricks are normally used in terms of cost. There are various kinds of fired bricks. Selection is made to use fired bricks having a desired property according to required functions. Among them, fired bricks that are excellent in corrosion resistance to molten glass are preferably used. In order to perform a thermal insulating and heat retaining function among the functions required to the backup, solid thermal insulating materials are preferably used.
Although the solid thermal insulating materials are satisfactory in terms of thermal insulating and heat retaining capacity, the solid thermal insulating materials are poor in corrosion resistance to molten glass in comparison with the fused cast refractories and fired bricks having an excellent corrosion resistance to molten glass among the fired bricks. For this reason, there is a risk that refractory bricks serving as the solid thermal insulating materials are significantly corroded by molten glass when the molten glass, which oozes out of joints between adjacent fused cast refractories constituting a conduit, has arrived at the solid thermal insulating materials constituting the backup. When solid thermal insulating materials constituting the backup are corroded, the vacuum degassing apparatus itself could have a shortened service life.
Patent Document 1 listed below discloses a conduit structure for molten glass which has a flow path formed in a polygonal shape in section, has joints disposed at the corners of the flow path with molten glass flowing at a low flow rate therein, and has cooling pipes disposed on the outer ends of the joints in order that the joints between adjacent refractory bricks, which are brought into direct contact with the molten glass, are prevented from being corroded to avoid the oozing-out of the molten glass from the joints.
In the proposal disclosed by Patent Document 1, the conduit structure is, however, necessarily complicated because it is necessary to dispose a cooling means, such as a cooling pipe, on each of the outer ends of the joints between adjacent refractory bricks. If water oozes out of a cooling pipe, there is a risk that refractory bricks are cracked by heat shock. There is also a risk that oozing cooling water contaminates the surroundings. Patent Document 1 discloses that unit bricks with a joint formed therebetween are provided at opposite ends with projections extending in an outer direction to extend the joints radially from the center of a flow path formed by the unit bricks in order to extend the length of the joints without increasing the thickness of the refractory bricks. When such projections are provided on the refractory bricks, there is, however, a risk that bricks are cracked because the temperature differences between an outer portion and an inner portion of each of the bricks is increased.
In order to solve the above-mentioned problems involved in the proposal disclosed in Patent Document 1, the inventors have disclosed, in Patent Document 2 listed below, a conduit structure for molten glass wherein refractory bricks are prevented from being cracked, without using a cooling means, such that the solid thermal insulating materials constituting the backup for a conduit are prevented from being corroded by molten glass oozing out of a joint between adjacent fused cast refractories constituting the conduit.
The conduit structure for molten glass disclosed by Patent Document 2 is characterized to be a conduit structure for molten glass wherein a conduit is constituted by fused cast refractories assembled together in a longitudinal and circumferential directions, and a backup disposed around the conduit, wherein the backup is constituted by a refractory layer disposed at an outer side of the conduit and a thermal insulating material layer, and wherein the fused cast refractories constituting the conduit and the refractory bricks constituting the refractory brick layer are selected such that the refractory layer contains a portion that has a temperature equal to the flow point of the molten glass when the molten glass passes through the conduit.
In the conduit structure disclosed by Patent Document 2, even when the molten glass oozes out of a joint between adjacent fused cast refractories constituting the conduit, the temperature of the molten glass lowers to a level of not higher than the flow point of the molten glass while the oozing-out molten glass is passing through the refractory brick layer. By this arrangement, there is no risk that the molten glass oozing out of a joint between adjacent fused cast refractories arrives at the thermal insulating material layer located at an outer position than the refractory brick layer. Thus, the solid thermal insulating materials constituting the backup can be prevented from being corroded by the molten glass that has oozed out of a joint between adjacent fused cast refractories.
The conduit structure disclosed by Patent Document 2 needs no cooling means, such as a cooling pipe, which contributes to prevent molten glass from oozing out. For this reason, the conduit structure does not need to be complicated. Further, there is no risk that water leakage out of the cooling pipe causes refractory bricks to be cracked, or that oozing cooling water contaminates the surroundings.
Furthermore, also in a glass melting furnace for melting a raw glass material and homogenizing and refining the molten glass, fused cast refractories are usually used as the refractory bricks disposed at a position in direct contact with the molten glass.
The glass melting furnace is required to have a thermal insulation structure in some cases. For example, when a dam wall is disposed so as to project from the bottom of a melting furnace forming a flow path for molten glass as in the glass melting furnace disclosed by Patent Document 3, the glass melting furnace is required to have an thermal insulation structure in order to control the corrosion of the dam wall by the molten glass. When a glass melting furnace is configured to have a thermal insulation structure, fired bricks are disposed as the refractory bricks outside the fused cast refractories, and solid thermal insulating materials are disposed outside the fired bricks in the same way as the above-mentioned conduit structure for molten glass.
The glass melting furnace thus configured involves a similar problem to the above-mentioned conduit structure for molten glass.